Question: Subtract.
Solution: There are many ways to solve this problem. Let's see two ways we can solve it. Place Value We can think in terms of tenths: $\begin{aligned} &\phantom{=}9.1 - 7\\\\ &=9.1 - 7.0\\\\ &=91\text{ tenths} - 70\text{ tenths}\\\\ &=21\text{ tenths}\\\\ &=2.1 \end{aligned}$ Place value table Let's line up the numbers by their decimal places. Ones $.$ Tenths ${9}$ $.$ $1$ ${7}$ $.$ $0$ Now, let's subtract each place value column. Ones $.$ Tenths ${9}$ $.$ $1$ $-$ $7$ $.$ $0$ $\underline{~~~~}$ $\underline{~~~~}$ $2$ $.$ $1$ $2.1 = 9.1-7$